1. Field of the Invention
This invention generally relates to a panel type recording device for use in conference and class rooms, and, in particular, to an electronic blackboard capable of automatically recording information written on a recording surface.
2. Description of the Prior Art
For many years, a writing panel, such as a blackboard, has been used as an aid in conference, teaching, etc. The writing panel may take the form of a blackboard, though the color may be often actually green, in which case chalk is used as a writing utensil to write information on the blackboard. The writing panel also may take the form of a white panel, in which case a marker containing an eraseable ink is used as a writing utensil. In either case, desired information can be written on the panel and the information can be erased for making the panel available for writing another information. However, the information written on the writing panel cannot be stored or recorded on a separate recording medium.
Under the circumstances, there has been developed a so-called electronic blackboard which is a recordable writing panel capable of recording information written on a writing medium. Such a recordable writing panel device is advantageous since it can produce the information written on the writing panel in the form of a hard copy and thus it is not necessary for somebody to rewrite the information on a separate sheet of paper. In such an electronic blackboard, use is often made of a Charge Coupled Device (CCD) image sensor for reading the information written on the writing medium so as to convert the visual information into an electronic signal. When such a CCD image sensor is used to read the information written on the writing medium, the image data read by the CCD image sensor must be subjected to threshold processing, whereby the analog image data is converted into a binary image data. For this purpose, a threshold level must be properly set.
FIG. 13 shows a typical prior art example for setting such a threshold level for use in converting the analog image data read by a CCD image sensor into a binary image signal. As shown, the prior art system includes a digital comparator 40 which receives at their terminals A0-A3 a digital data, such as 4-bit data, which corresponds to the level of an image data supplied from a CCD image sensor. This digital data is compared with the threshold of the digital comparator 40 to carry out the binary conversion processing of the digital data. In this prior art system, a plurality of switches 41 are provided each connected to the corresponding one of another set of input terminals B0-B3 of the digital comparator 40, and the plurality of switches 41 are manually operated to set the threshold level of the digital comparator 40 to be used for thresholding operation.
However, when the CCD image sensor has been replaced with a new one, or an illumination lamp for illuminating the surface of the writing medium has been replaced with a new one or has deteriorated, the condition for reading the surface of the writing medium changes, which requires to reset the threshold level of the digital comparator 40. In the case of the prior art system shown in FIG. 13, the information on the writing medium must first be recorded on a recording material to see whether or not the reading condition for the CCD image sensor is appropriate or not. Then, based on the quality of the recorded image, the switches 41 may be appropriately operated to change the threshold level of the digital comparator. And, this process is repeated as many times as desired until a desired reading condition is obtained. It is apparent that such a manual adjustment process is cumbersome and time-consuming and it often requires skills and experiences.